The cutting processing of plate-like workpieces, e.g., metal sheets, can be effected by thermal processing such as laser beam cutting, or by mechanical processing, e.g., by stamping processing (nibbling). During laser processing, the cutting is carried out by a laser processing head that is moved along a predefined cut contour relative to a workpiece arranged in a processing plane. To produce the relative movement, the laser processing head and/or the workpiece can be displaced in the processing plane. During cutting of a workpiece by stamping processing, the stamping process is generally performed on a stationary stamping press, the workpiece being displaced in the processing plane. Both stamping and thermal processing can be performed on one and the same machine tool.
During cutting processing, the workpiece is separated along a (typically closed) cut contour into one or a plurality of parts of the workpiece and a remainder of the workpiece (residual part or residual lattice). The parts of the workpiece fall from the residual lattice upon being completely cut free and can, if they are parts of acceptable quality, be supported on supporting elements arranged below the residual lattice. However, the parts of the workpiece that have been cut free can also be waste that is disposed of after being cut free.
During the process of separating by cutting, individual parts of the workpiece, in particular cutting swarf (or filings or chips), on account of process faults or incorrectly set process parameters, may not be completely cut free or possibly not cut free at all from the residual lattice. These incorrectly cut parts of the workpiece therefore do not fall from the residual lattice, with the result that a complete perforation in the residual lattice does not arise during cutting processing within the desired cut contour. Such a fault is generally not detected during automated manufacture and may possibly be identified by an operator or some other process participant only during subsequent quality inspection. In this context, cutting swarf constitutes parts of the workpiece which are cut out from parts of acceptable quality to produce the contour as desired and are predominantly disposed of as waste.
U.S. Pat. No. 4,504,727 describes a system for controlling laser processing on a circuit board. During drilling processing through a plurality of layers composed of different layer materials, with the aid of a photoacoustic signal generated by the pulsed laser beam in the respective layer material, a cutting end point can be identified and a cutting fault can be avoided.
EP 1 886 757 A1 describes an apparatus and a method for online monitoring of the quality of a laser process on a workpiece by a thermal imaging camera. The thermal imaging camera detects radiation from a zone of interaction of the laser beam with the workpiece. On the basis of the thermal image, it is possible to deduce, for example, incomplete severing of the workpiece during a cutting process.
A laser processing machine for detecting a cutting fault during a cutting process has also been disclosed in JP 03077790 A. A photodetector arranged below the workpiece in the region of a cut gap detects laser radiation passing through the cut gap. If the photodetector no longer detects radiation, the workpiece was not completely severed during laser cutting, and so a cutting fault is present.
JP 2002 331 383 A describes a device for monitoring a thermal cutting process, in which an actual cut gap width is compared with a desired cut gap width to determine a cutting fault. To determine the actual cut gap width, an apparatus rotating around a flame cutting device is provided, which apparatus allows observation of the cut gap behind the flame cutting device.
DE 691 14 399 T2 has disclosed an apparatus which makes it possible to detect the end of piercing or severing during the cutting of metal surfaces. This apparatus has a sensor head for detecting light generated at a surface of a (metallic) workpiece during the laser processing. The apparatus detects the end of severing during a piercing process on the basis of a signal drop of the detected signal. Subsequently, a cutting process is initiated and a reference voltage value of a voltage derived from the signal is used to determine whether the cutting process proceeds correctly or erroneously.